The present invention relates to a sensing device for detecting various kinds of physical quantities, and more particularly to a sensing device having a function for adjusting output characteristics of the sensing device or a control state of a sensing element.
As a prior art of adjusting output characteristics of a sensing device by applying an electric signal to an adjusting terminal, an acceleration sensor as disclosed in Japanese Patent Application Laid-Open No. 5-340958 is raised. The acceleration sensor has an acceleration detection processing circuit provided with a sensitivity adjusting unit and a zero-point adjusting unit, and an adjusting unit capable of varying a basic output characteristic of sensitivity. The adjusting unit is constructed by combination of a plurality of resistors and switches, and on-off operation of the switch is performed by on-chip trimming from external.
In the prior art described above, the adjusting terminals capable of performing on-off of the adjusting switches are provided on an electronic circuit board, and in a fabrication process of the product there are a filling process of a gel into a housing having the circuit board inside after adjusting the output characteristic and heating processes for bonding a cover and setting the gel and the cover adhesive. Variations of output characteristics in the final products are caused by variations of output characteristics by the process.
Further, in an air flow measuring instrument for an internal combustion engine, in order to make it small in size, light in weight and low in cost, it is preferable that an air flow sensing unit is integrated in a housing containing a circuit board inside and the housing is inserted and mounted so that the sensing device is positioned in an air intake flow passage of the internal combustion engine. Since the adjusting work needs to be performed by actually flowing air through the flow sensing unit, the air flow measuring instrument is mounted on a rig corresponding to the air intake flow passage after almost completing the housing and the flow sensing unit, and adjusting work is performed from the outside of the rig by actually flowing air. A means capable of performing such adjusting work is necessary.
Particularly, in a thermal type air flow measuring instrument, the sensing element can be made small in size and the sensing element needs to be electrically connected to an electronic circuit for controlling the sensing element and outputting. Therefore, it is possible to provide a small-sized and low-cost air flow measuring instrument in which a flow sensing unit is integrated in a housing containing an electronic circuit. However, since the electronic circuit is also placed in the air intake flow passage, adjusting work is performed from the outside of the air intake flow passage when the adjusting work is performed by actually flowing air through the flow sensing unit. A means capable of performing such adjusting work is necessary. Further, in a case where the sensing element is placed in an auxiliary passage from the viewpoint of various kinds of performance and reliability, in taking it into consideration to make the instrument small in size and low in cost, the number of parts is reduced by forming the auxiliary passage in the housing and forming the auxiliary passage by the cover of the housing. However, in taking into consideration the effect of variations in shapes of the auxiliary passage on the output characteristics, it is necessary to perform adjustment by actually flowing air after completion of the auxiliary passage, and a means capable of adjusting from external is further required.